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"Chassis copper pours" in offline SMPS


Advanced Member level 5
Jun 13, 2021
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If you have any offline SMPS which exists in an earthed metal enclosure, then you should (preferably) incorporate proper EMI grounding, as in the pages of the below Vicorpower document. (ie, the yellow bit of copper pour under the isolated DCDC module, which is explained throughout the document) Otherwise you will suffer poorer Common mode emissions performance.

So, in an offline SMPS PCB, in an internal copper layer, right under the isolation transformer…you should really put a “chassis” copper pour which is either directly connected to the chassis somewhere….or, you can capacitively couple this “chassis copper pour” to the earthed metal enclosure with Y caps.

As explained in the document, this is needed as it acts as a shield….and catches the common mode emissions, and then allows them to be chanelled through the y capacitors, and back into the Line/Neutral, where of course, they do not manifest, as common mode emissions.

This is clearly seen in the below vicorpower document….however, we don’t often see it in offline SMPS PCBs…why is this?

Clearly, the “chassis copper pour”, beneath the transformer, is needed, so as to stop the emissions escaping the PSU to the surrounding earth…….and manifesting as a big common mode emissions problem.

Though Most people just shovel in a few Y caps and a Common Mode Choke....but if you have an earthed metal enclosure...this is how you can get improved common mode emissions performence.
Because we all know, that Earthed metal enclosures are a common mode emissions hazard...since common mode emisisons can couple easily to the surrounding earthed metal enclosure.
So, with a cheaopinternal chassis copper can mitigate Common mode problems.

Vicorpower modules common mode filtration (do search on “EMI” within this doc)

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